1. Field of the Invention
The invention relates to composite membranes that are formed by casting a polymeric solution onto a porous nonwoven substrate thereby forming a composite of a microporous membrane layer and a support layer. The composite membranes are especially suited for use in fluid filtration, including the filtration of liquids and gases.
2. Description of the Related Art
The filtration of liquids includes the general categories of “microfiltration,” in which particulate matter in the range of about 0.1 μm to about 10 μm in diameter is filtered from the liquid, “ultrafiltration,” in which matter in the range of about 50 nm to about 0.5 μm in diameter is filtered, and reverse osmosis in which matter in the range of about 1 Å to about 1 nm in diameter is filtered. Microfiltration, ultrafiltration and reverse osmosis each utilize a polymeric microporous membrane as the filtration medium, either used alone or in conjunction with a support layer. When used in conjunction with a support layer, a composite membrane is formed. The support layer provides the mechanical integrity of the composite membrane by virtue of its strength and stiffness, without reducing the permeability of the composite membrane. The support layer is a substrate onto which the polymeric solution forming the microporous membrane is cast and solidified, thus forming the composite membrane. In the case of ultrafiltration, the filtration membrane is typically cast onto a polypropylene or polyethylene substrate. In the case of reverse osmosis membranes, the microporous membrane is typically cast onto a polyester substrate.
The performance of composite membranes is negatively impacted by a number of deficiencies encountered with currently available composite membranes. For one, the composite membrane can delaminate due to poor adhesion between the microporous membrane and the support layer, which in turn may be attributed to poor affinity of the microporous membrane for the support layer or poor mechanical interlocking at the interface of the support layer and the microporous membrane. When the liquid membrane-forming polymeric solution is first cast onto the support layer, the solution penetrates beyond the surface of the support layer and subsequently solidifies around the fibers at and near the surface of the support layer, providing mechanical interlocking between the microporous membrane and the support layer. Delamination of the microporous membrane and the support layer during use of the composite membrane results in non-uniformity of the composite membrane and compromised filtration performance.
Another problem encountered with conventional composite membranes is small discontinuities in the microporous membrane or “pin holes” caused by defects in the surface of the casting substrates such as individual raised or standing fibers. By “raised fibers” is meant free ends or raised loops of fibers of the substrate that stand out of the plane of the substrate surface. Raised fibers break the continuity of the surface of the substrate, such that it is difficult or impossible to cast a continuous, uniform membrane.
Another problem which has been encountered with conventional composite membranes during the casting process is “strikethrough” of the membrane polymer solution, in which the polymer solution passes through the thickness of the substrate, resulting in nonuniform permeability and filtration efficiency. Strikethrough occurs in substrates at areas having high permeability due to large pores and/or increased number of pores (resulting in high porosity). Large pores often occur in nonwoven substrates having coarse or large diameter fibers. Conversely, the use of substrates having low permeability due to very fine fibers and small mean pore sizes on the surface thereof may result in insufficient penetration of the membrane solution, or in insufficient filtration capacity of the composite membrane.
Another problem encountered in conventional composite membranes is a high degree of variability of the thickness of the substrate which results in variability in the thickness of the microporous membrane which is cast thereon, again resulting in nonuniform performance of the composite membrane.
U.S. Pat. Nos. 4,728,394 and 4,795,559 disclose a nonwoven support layer for casting semipermeable membranes thereon, comprising a laminate of a low density layer made entirely from air-laid or carded polyester fibers containing 20 to 80 % undrawn polyester or bicomponent polyester fibers, and a high density layer.
U.S. Pat. No. 5,989,432 discloses a composite membrane including a semipermeable membrane, a support layer and a nonwoven web of multicomponent fibers therebetween wherein the multicomponent fibers comprise a first polymer as the core component and a second polymer on the surface of the fibers, the second polymer having a softening temperature below the softening temperatures of the first polymer, the membrane and the support layer.
U.S. Patent Publication No. 2004/0045892 A discloses an asymmetric nonwoven support layer having a microporous casting layer having a mean pore size no greater than about 300 μm on the surface thereof for casting semipermeable membranes, the casting layer formed by heat treating, calendering, melt-blowing or wet-laying a layer of fibers.
There is a need for an improved substrate onto which a microporous membrane is cast for forming composite membranes, the substrate having improved pore size uniformity and surface smoothness such that a membrane of uniform, controllable thickness and substantially free of pin holes is formed when cast thereon. It would also be desirable to have an improved substrate that would provide improved adherence of the microporous membrane to the substrate.